1. Field of the Invention
The present invention generally relates to processes for producing synthesis gas from light hydrocarbons employing a short contact time reactor. More particularly, the invention pertains to methods of increasing the yield of syngas in processes employing partial oxidation of methane or natural gas to products containing CO and H2 by concurrent catalytic partial oxidation of H2S to elemental sulfur and hydrogen.
2. Description of Related Art
Many refineries face an abundant supply of lower alkanes, i.e., C1-C4 alkanes such as methane, and relatively few means of converting them to more valuable products. Moreover, vast reserves of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. There is great incentive to exploit these natural gas formations, however most natural gas formations are situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported than syngas.
The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes. Syngas generation from methane typically takes place by one of three techniques. Steam reforming of methane is the most common, followed by partial oxidation, and autothermal reforming.
The partial oxidation of methane can be represented by the reaction shown in equation (1):
CH4+xc2xdO2xe2x86x92CO+2H2xe2x80x83xe2x80x83(1)
At the same time, some of the methane bums completely, according to equation (2):
CH4+2O2xe2x86x92CO2+2H2Oxe2x80x83xe2x80x83(2)
Hence, syngas is typically a mixture of carbon monoxide and molecular hydrogen, generally having a hydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and may contain other gases such as carbon dioxide. Synthesis gas is not usually considered a commodity; instead, it is typically generated on-site for further processing. Synthesis gas is commonly used as a feedstock for conversion to alcohols (e.g., methanol), olefins, or saturated hydrocarbons (paraffins) according to the well-known Fischer-Tropsch process, and by other means. The resulting high molecular weight (e.g. C50+) paraffins, in turn, provide an ideal feedstock for hydrocracking, a feedstock for conversion to high quality jet fuel, and superior high octane value diesel fuel blending components.
Emerging technologies that have been developed to generate syngas from methane include a technique that entails exposing a mixture of methane and oxygen to a hot catalyst for a brief time, typically on the order of milliseconds, followed by cooling of the resultant gas stream. EPO Patent No. 303,438 describes a process for synthesis gas production by catalytic partial oxidation to overcome some of the disadvantages and costs of steam reforming. A monolith catalyst is used with or without metal addition to the surface of the monolith and the process operates at space velocities of 20,000-500,000 hrxe2x88x921. Conventional catalytic partial oxidation processes are also described, for example, in U.S. Pat. Nos. 5,654,491, 5,639,929, 5,648,582 and in J. Catalysis 138, 267-282 (1992), the disclosures of which are incorporated herein by reference. Although in conventional short contact time syngas generation systems the syngas reaction can be self-sustaining once initiated, it has been shown that 10-15% of the carbon initially present as methane can be lost to the formation of CO2 in combustion via equation (2), above. This directly reduces the yield of syngas that can be obtained. Therefore it is desirable to use a syngas generation system that allows a better yield of carbon monoxide and hydrogen.
Further complicating the exploitation of the world""s natural gas supply is the fact that many natural gas formations contain H2S in concentrations ranging from trace amounts up to about 3-25% (by volume) hydrogen sulfide. For example, many of the catalysts that are conventionally used for the production of synthesis gas are poisoned by the presence of sulfur.
If the hydrocarbon conversion does proceed to some degree, the syngas product is typically contaminated by passed through H2S and/or SO2. The presence of H2S or SO2 generally diminishes the usefulness of the syngas or creates environmental safety concerns. It would be highly desirable in the natural gas exploitation industry to find a way to efficiently convert the light hydrocarbon content of the natural gas to synthesis gas without conducting an initial sulfur removal operation. In a related aspect of petroleum refining, some petroleum feed streams and separated fractions contain sulfur. Sulfur is typically undesirable in most petroleum refining processes and products. Refineries typically upgrade the quality of the various petroleum fractions by removing the sulfur before they are processed further. Hydrodesulfurization units are used to break down the sulfur compounds in the petroleum fractions and convert the sulfur to H2S. In addition to hydrodesulfurization processes, other conversion processes in a typical refinery, such as fluid catalytic cracking, coking, visbreaking, and thermal cracking, produce H2S from sulfur containing petroleum fractions. The H2S from both the desulfurization processes and these conversion processes is typically removed from the gas streams or light liquid hydrocarbon streams using either chemical solvents based on alkanolamine chemistry or physical solvents. A circulating, regenerative H2S removal system employing an absorption stage for H2S pickup and a regeneration stage for H2S rejection produces a concentrated stream of H2S.
In conventional systems, this H2S stream is then fed to a H2S conversion unit, which converts the H2S into a storable, saleable product such as elemental sulfur, sodium hydrosulfide solution, or sulfuric acid. Conversion of the H2S to elemental sulfur is most common, mainly because elemental sulfur is the most marketable sulfur compound of those typically produced.
The process most commonly used to recover elemental sulfur from H2S gas is the modified Claus sulfur recovery process. The conventional Claus process is well known in the art, and is also described in U.S. Pat. application Ser. No. 09/624,715, the disclosure of which is incorporated herein by reference.
U.S. Pat. No. 5,720,901 describes a process for the catalytic partial oxidation of hydrocarbons in which nitrogen is present in the hydrocarbon feed mixture. An organic or inorganic sulfur-containing compound is present in the feed mixture in a sufficient concentration (i.e., 0.05 to 100 ppm) to reduce the presence of nitrogen by-products, particularly ammonia and hydrogen cyanide, in the products of the catalytic partial oxidation process. Hydrocarbon feedstocks used directly from naturally occurring reservoirs in which the sulfur content is significantly above the aforementioned limits may be subjected to a partial sulfur removal treatment before being employed in that process. A sulfur removal step is applied to the product stream if the carbon monoxide and/or hydrogen products of the process are to be utilized in applications that are sensitive to the presence of sulfur, such as Fischer-Tropsch synthesis.
It would be desirable to have a syngas production process than can avoid the need for an initial sulfur-removal step from H2S-containing natural gas sources. It would also be desirable to have a syngas production process with improved yield and selectivity for CO and H2 products compared to conventional syngas processes. Also needed are new and better ways to utilize H2S gas streams arising from existing desulfurization processes.
The present invention provides a method, system and catalysts that improve the yield of syngas generation and selectivity for CO and H2 products, at least in part by substituting H2S partial oxidation for methane combustion in a syngas reactor. The partial oxidation of H2S provides the heat necessary to sustain the syngas reaction at the desired temperature without consuming the methane or other light hydrocarbon. Hence, less methane is lost to complete combustion and yield of the product is increased. In accordance with certain embodiments of the invention, a process for producing synthesis gas is provided. The process comprises, contacting an H2S-containing light hydrocarbon stream, in the presence of O2, with a catalyst having activity for catalyzing the partial oxidation of the hydrocarbon to a product comprising CO and H2 and also having activity for catalyzing the partial oxidation of H2S to elemental sulfur and water, under reaction promoting conditions of temperature, flow rate, molar ratios of reactant gases, and reactant gas/catalyst contact time. The process also includes maintaining the reaction promoting conditions such that the reactions
H2S+xc2xdO2xe2x86x921/x Sx+H2O, where x equals 2, 6, or 8, and (3)
CH4+xc2xdO2xe2x86x92CO+2H2xe2x80x83xe2x80x83(1)
simultaneously occur and a process gas stream is obtained comprising CO, H2, gaseous elemental sulfur and steam. The process further comprises condensing elemental sulfur from the process gas stream to provide a substantially desulfurized synthesis gas stream. In preferred embodiments the process includes contacting the catalyst with a portion of the H2S-containing light hydrocarbon stream for no more than about 10 milliseconds.
In some embodiments, a process for producing synthesis gas is provided that comprises providing a first gas stream containing a C1-C4 alkane, or mixture thereof, and, optionally, H2S. Optionally, a second gas stream containing H2S mixed with the first gas stream, such that a H2S-containing hydrocarbon feed gas stream is produced having a H2S:CH4 molar ratio of about 1:10 to about 2:3. The process includes mixing the H2S-containing hydrocarbon feed gas stream with an O2-containing stream to form a reactant gas stream, the reactant gas stream having a CH4:O2 molar ratio of about 1.5:1 to about 2.2:1. The process also includes passing the reactant gas stream over a catalyst such that a portion of the reactant gas contacts the catalyst for no more than about 10 milliseconds. The selected catalyst is capable of catalyzing the partial oxidation of to CO and H2 and also capable of catalyzing the partial oxidation of H2S to elemental sulfur and water under reaction promoting conditions. The process further includes maintaining reaction-promoting conditions of temperature, molar ratios of reactant gas components, and flow rate such that a gaseous product stream comprising CO, H2, 1/x Sx and H2O is obtained, wherein X=2, 6, or 8. The gaseous product stream is then cooled to the condensation temperature of elemental sulfur, or lower, such that elemental sulfur condenses from the product stream and an at least partially desulfurized gaseous product stream is obtained. Optionally, elemental sulfur product is recovered. Any residual gaseous sulfur-containing components may, optionally, be removed from the at least partially desulfurized gaseous product stream using, for example, a sulfur absorbing sulfur absorbing material such as zinc or iron oxide. Substantially sulfur-free synthesis gas, preferably in high yield, having a H2:CO molar ratio of about 2:1 and containing less than about 10 vol. % CO2 is recovered from said product stream.
Also provided according to certain embodiments of the invention is a method for improving the yield of a syngas generation system, comprising providing a first gas stream containing a light hydrocarbon, mixing a second gas stream containing H2S with the first gas stream to form a feed gas stream, mixing the feed gas stream with an oxygen containing stream to form a mixed feed stream, contacting the mixed feed stream with a hot catalyst to form a product stream, and removing syngas and elemental sulfur from the product stream. In certain embodiments, the method further comprises removing residual H2S from the product stream.
According to some embodiments the step of mixing a second gas stream comprising H2S with the first gas stream to form a feed gas stream is carried out at temperatures up to about 300xc2x0 C. In some embodiments the step of contacting the feed gas stream with a hot catalyst to form a product stream is carried out at temperatures above 500xc2x0 C., preferably between about 850 and 1,500xc2x0 C.
In some embodiments the method of improving syngas yield includes converting less than 10 vol. % of the light hydrocarbon to carbon dioxide. In preferred embodiments the catalyst contact time is less than 10 milliseconds.
According to some embodiments, the method employs a catalyst is selected from the group consisting of: platinum, rhodium, iridium, nickel, palladium, iron, cobalt rhenium rubidium, Pdxe2x80x94La2O3, Pt/ZrO2, Pt/Al2O3 and combinations thereof.
Another aspect of the present invention provides a system for the partial oxidation of light hydrocarbons, comprising a hydrocarbon injection line, an H2S injection line in communication with said hydrocarbon injection line, an oxygen injection line in communication with said hydrocarbon injection line, a reaction zone receiving gases from said hydrocarbon, H2S and oxygen injection lines and including a catalyst suitable for catalyzing said hydrocarbon to form CO and H2. In some embodiments the system includes a mixing zone upstream of the reaction zone, the mixing zone receiving gases from the hydrocarbon and the H2S lines, wherein the temperature of the mixing zone is up to about 300xc2x0 C. In certain embodiments the system comprises a thermal barrier between the mixing zone and the reaction zone. In certain embodiments the oxygen injection line communicates with the reaction zone, and in some embodiments the mixing zone receives oxygen from said oxygen injection line. According to some embodiments the temperature of the reaction zone is between about 850-1,500xc2x0 C. Preferably the system includes at least one cooling zone downstream of the reaction zone, and at least one tailgas processing unit downstream of the final cooling zone. In some embodiments the catalyst is supported on wire gauze and comprises platinum, rhodium, iridium, nickel, palladium, iron, cobalt, rhenium, rubidium, Pdxe2x80x94La2O3, Pt/ZrO2, Pt/Al2O3, or a combination thereof.
In yet another aspect of the present invention is provided a method for improving the yield of a syngas generation system. The method comprises providing a gas stream comprising a light hydrocarbon, mixing a second gas stream comprising H2S with the first gas stream to form a feed gas stream, while maintaining the temperature of the feed gas stream below about 500xc2x0 C., and preferably no more than about 300xc2x0 C. The method also includes contacting the feed gas stream with a hot catalyst, in the presence of O2, to form a product stream wherein less than 10% of the carbon atoms in the light hydrocarbon is converted to carbon dioxide, and the removing syngas and elemental sulfur from the product stream. In some embodiments the method comprises mixing O2 with the light hydrocarbon prior to contacting the feed gas stream with a hot catalyst. In some embodiments the O2 is mixed with the light hydrocarbon during the contacting of the feed gas stream with a hot catalyst.
Yet another aspect of the present invention provides a catalyst having activity for concurrently catalyzing the partial oxidation of a light hydrocarbon and for catalyzing the partial oxidation of H2S to produce a product mixture comprising CO, H2, 1/x Sx and H2O, wherein x=2, 6 or 8, under reaction promoting conditions of temperature, flow rate, molar ratios of reactant gases, and reactant gas/catalyst contact time up to about 10 milliseconds. Preferred catalyst compositions comprise platinum, rhodium, iridium, nickel, palladium, iron, cobalt, rhenium or rubidium, or a combination of any of those metals. Preferably, a lanthanide element or oxide thereof is also included. The catalyst may also be supported on a refractory support, having sufficient strength and transparency to permit high pressures and flow rates, and made of alumina, zirconia or partially stabilized (MgO) zirconia, for example. In some embodiments the catalyst comprises Pdxe2x80x94La2O3. In some embodiments the catalyst comprises Pt/ZrO2 or Pt/Al2O3. In some embodiments the catalyst comprises about 87-93 wt % Pt and about 7-13 wt % Rh.
In certain preferred embodiments, the catalyst comprises rhodium and samarium on an alumina or partially stabilized (MgO) zirconia (PSZ) support. According to one such embodiment the catalyst is prepared by depositing about 4-6 wt % Rh onto a layer of about 5 wt % Sm coating a PSZ monolith. These and other embodiments, features and advantages of the present invention will become apparent with reference to the following drawings and description.